Jidoka ("autonomation," or automation with a human touch) gives machines and operators the ability to detect an abnormal condition and stop immediately, so a defect is never built into the next unit or passed downstream. The line stops, the problem is made visible (via andon), and the root cause is fixed.
Together with Just-in-Time, jidoka is one of the two pillars of the Toyota Production System. It builds quality into the process rather than inspecting it in afterward.